Regulated dc-dc power supply

ABSTRACT

An improved direct-current power supply is provided for converting alternating-current power or direct-current power into direct-current power, and which has particular utility in energizing electronic equipment such as computers, data processors, and the like. The power supply of the invention is a regulated fly-back type in which electric energy from an appropriate source is alternately stored in an electromagnetic device, such as a transformer, and then released into a load. The improved power supply of the invention includes control circuitry which causes a constant amount of energy to be stored during each cycle independently of source voltage, and which enables the power supply to exhibit highly favorable regulation characteristics through a wide range of loads.

United States Patent Berger [4 1 Oct. 10, 1972 [54 REGULATED DC-DC POWERSUPPLY 3,435,320 3/1969 I Lee et al ..32l/2 [72] Inventor; James Berger,Sherman Oaks, 3,526,823 9/1970 Genuit ..321/2 Calif. PrimaryExaminer-William H. Beha, Jr. [73] Assignee: Pioneer Magnetics, Inc.,Santa A;; i h D B h r Monica, Calif. 22] Filed: Jan. 27, 1972 [57]ABSTRACT [211 App]. No: 221,272 An improved direct-current power supplyis provided for converting altemating-current power or direct-cur-Related US. Application Data rent power into direct-current power, andwhich has particular utility in energizing electronic equipment [63]5833:5 2 of July 1970 such as computers, data processors, and the like.The power supply of the invention is a regulated'fiy-back type in whichelectric energy from an appropriate [52] US. Cl...321/%235ig4,3332lv/l111i Source is alternately stored in anelectromagnetic l 1 Int Cl 02m 3/32 H03k 3B0 device, such as atransformer, and then released into a l 58] Fie'ld 4 18 320/1, load. Theimproved power supply of the invention in- 5 cludes control circuitrywhich causes a constant amount of energy to be stored during each cycleindependently of source voltage, and which enables the [56] ReferencesCited power supply to exhibit highly favorable regulation UN S S PATENTScharacteristics through a wide range of loads.

3,575,153 4/1971 Hardin et a] ..32l/2 X 4 Claims, 5 Drawing Figures Arm!C0 I1 I70 I H a 6? f 1 002x Q ar ,5 we FIE/Cm; m2 (@212 Aw; 002 mm; Q2Q3 2 (10? 62105 K Q4 0* K 1 ll 4242 4205 fizfli 115m: M :2 r r F (0205(my I an: a; 020! czzm i 4205 4 207 e 1 2a; 202 2- @202 (:02 '1 V2034209 k/ 106 i may F 02 05 H l 4M 5421! I I JZ/VZZIIDZ-JZ- 215 pg]; I 214Sal/Awe V04 7466 uric/M- SHEET 1 0F 4 m. .QWF

PATENTEUnm 10 I972 PATENTEDHIJI 10 I972 sum 2 or 4 REGULATED DC-DC POWERSUPPLY This application is a continuation of copending application Ser.No. 58,042, filed July 24, I970, now abancloned.

BACKGROUND OF THE INVENTION The improved and unique direct-current powersupply of the present invention is intended to replace the usualrelatively heavy, costly and inefficient prior art power supply. Forexample, a power supply constructed in accordance with the concepts ofthe present invention has been built having a weight less than onethirdthe weight of an equivalent prior art power supply; and of approximatelytwo-thirds the size of the prior art power supply, and at a lower cost.

A feature of the power supply of the present invention which hasparticular utility when used in computer applications, for example, isits capability of maintaining power output for an appreciable time afteran alternating-current power failure has occurred. For example, aconstructed embodiment of the invention has maintained power for morethan 20 milliseconds at full load aftersuch a power failure, as comparedwith a usual 2 millisecond hold time in the prior art systems. This holdinterval gives the computer time to clear the information beingprocessed into memory so that it is not lost.

The improved power supply of the invention, in the embodiment to bedescribed, is controlled, as mentioned above, to provide a fixed amountof energy storage for each cycle independent of the source voltage forregulation purposes. The power supply system to be described alsoprovides overload and short-circuit protection. Moreover, the powersupply is such that it may be activated under full load conditions.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram partlyin block form and partly in circuit detail representing a systemconstructed to incorporate the concepts of the present invention;

FIG. 2 is a circuit diagram of a pulse generator which may be includedin the system of FIG. 1;

FIG. 3 is a circuit diagram of an inhibit control circuit which also maybe included in the system of FIG. 1;

FIG. 4 is a circuit diagram of a voltage source and source voltagedetector circuit which likewise may be included in the system of FIG. 1;and

FIG. 5 is a circuit diagram of a power supply system representative ofone embodiment of the invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT The power supplysystem of the invention, as mentioned above, is a fly-back regulatedtype. The system includes a voltage source 20 which is connected to theprimary winding N, of an electromagnetic device, such as a transformerT,, through a switching transistor designed Q in FIG. 1. The secondaryN, of the transformer T, is connected to an appropriate load designatedR. A capacitive element C is also connected across the secondary N Apower diode designated CR is incorporated between the secondary N, andthe com mon or ground side of the capacitive element C and load R.

The aforesaid power diode CR is connected to an inhibit control circuit14 which, in turn, is connected back to a pulse generator 12. A sourcevoltage detector 16 is also connected to the output of the voltagesource 20 and back to the pulse generator 12. The secondary N of thetransformer T, is also connected to a pre-amplifier 10 which, in turn,is connected to the pulse generator 12. The pulse generator 12 iscoupled through an appropriate coupling network 18 to the switchingtransistor Q.

The pulse generator 12 produces a train of pulses on lead D, and thesepulses are amplified in the coupling network 18 and applied through asuitable impedance matching and isolating network to the lead designatedE, and by that lead to the base of the switching transistor Q. Thepulses generated by the pulse generator 12 have a suitable duration of,for example, 30 microseconds, so as to allow a relatively large currentof, for example, 12 amperes to flow in the primary winding N, of thetransformer T,. This primary current is zero when the transistor 0 isfirst made conductive, and it increases linearly with time according tothe magnitude of the voltage from the source 20, divided by theinductance of the primary winding N, of the transformer T,. At the endof each pulse from the pulse generator 12, the switching transistor 0 isturned off, and the energy which has been stored in the transformer T,is then released into the load R through the power diode CR.

The source voltage detector 16 provides a control signal on the lead Bwhich is proportional to the source voltage, and this signal is appliedto the pulse generator 12. The pulse generator contains a network whichvaries the duration of the generated pulses in an inverse proportionalrelationship with the amplitude of the control signal from the detector16. This causes the product of the duration of the individual pulsesgenerated by the pulse generator and the amplitude of the voltage fromsource 20 to be maintained constant. Since the current flowing in theprimary winding N, of the transformer T, at the time of each turn-off ofthe switching transistor Q is proportional to the product of the pulsewidth and amplitude of the source voltage, the current will then be thesame at the end of each pulse and hence the energy stored will be thesame at the end of each pulse, regardless of the source voltageamplitude, and providing that the energy stored at the beginning of eachpulse is zero.

The inhibit control circuit 14 senses whether the power diode CR isconducting, and prevents pulses from occurring until the diode CR isnon-conductive. Since, when the switching transistor Q is renderednonconductive, any current existing in the transformer T, must flowthrough the diode CR, the inhibit control circuit 14 indicates when theenergy in the transformer T, has been reduced to zero. This is achievedby sensing the point at which current no longer flows in the power diodeCR.

Therefore, the inhibit control circuit 14 prevents the pulse generator12 from generating a pulse until all the energy stored in thetransformer T, by the preceding pulse has been released. This means thatthe current build-up in the transformer T, can never be cumulative fromcycle to cycle, and there is no possibility for the switching transistor0 to be destroyed.

The pre-amplifier circuit 10 senses the amplitude of the output voltageof the power supply, and this circuit prevents the pulse generator 12from generating pulses when the output voltage is above apre-established threshold. in this way, regulation of the output voltageis accomplished. During normal operation of the power supply system,when the output voltage decays due to dis-charge of the output filtercapacitor C by the load R below the pre-established level, a pulse isinitiated by the pulse generator 12.

During each pulse from the pulse generator 12 the transistor is switchedon, and energy is stored in the transformer T, as the output voltagecontinues to decay. At the end of the pulse, the switching transistor Qis rendered non-conductive and energy is released into the load filtercapacitor C, through the power diode CR. The latter energy is sufficientto charge the filter capacitor C to above the pre-established threshold.When the output voltage again decays to the preset threshold, a newpulse is initiated from the pulse generator 12, but not before all theenergy from the previous pulse has been released, due to the action ofthe inhibit control circuit 14 in sensing current flow in the powerdiode CR.

From the foregoing description, it will be appreciated that the energyin each pulse is maintained constant, and that the repetition rate ofthe pulses is governed by the load demand. Specifically, the pulsesgenerated by the pulse generator 12 and stored in the transformer T,will occur just often enough to supply the output demand, and voltageregulation is thereby achieved When overload conditions occur, theoutput voltage becomes less than the pre-established threshold. Undersuch conditions, a new pulse will be generated by the pulse generator 12as soon, as but not before, the energy in the transformer T, is entirelyreleased, thus providing maximum possible output power. As the Overloadis increased, and the output voltage correspondingly reduced, therepetition rate decreases, because more time is required to dischargethe transformer T, after each pulsefrom the pulse generator. Thisresults in a limiting of the output current, which protects thecomponents of the system.

It is conceivable that a slight negative loading of the power supplyfrom some external source could cause a continuous flow of currentthrough the power diode CR prior to the system being turned on. Undersuch conditions, the system would normally never initiate a pulse due tothe inhibit control circuit 14 continuously sensing current in the powerdiode CR. To prevent such a situation, the inhibit control circuit is ACcoupled to a DC restoration circuit, so that it operates in the desiredmanner during normal operating frequencies, but cannot inhibit the pulsegenerator 12 for any prolonged interval in the event of a continuouscurrent in the power diode CR.

The pulse generator 12 may have a circuit configuration such as shown inFIG. 2. The circuit includes, for example, an NPN transistor 0209 whosecollector is connected to the lead C from the inhibit controlcircuit l4,and whose emitter is grounded. The base of the transistor 0209 isconnected to the lead A from the pre-amplifier l0, and to a grounded 3.3kilo-ohm resistor R214. The collector of the transistor 0209 isconnected to the collector of a PNP transistor 0201 and to the base ofan NPN transistor 0205. The emitter of the transistor 0205 is grounded,and its collector is connected to the base of the transistor 0201 and toa 15 kiloohm resistor R205. The resistor R205 is connected to thepositive terminal of a l5-volt auxiliary voltage source.

The collector of the transistor 0201 is also connected to the base ofthe transistor 0205 and to a 680 ohm resistor R210. The resistor R210 isconnected to the junction of a Zener diode CR207 and a grounded 680 ohmresistor R211. The Zener diode is connected to a diode CR202 which, inturn, is connected to the emitter of the transistor 0201. The zenerdiode is also connected to a 4.7 kilo-ohm resistor R204 and to a 0.01microfarad capacitor C202 and to a diode CR206. The resistor R204 isconnected to the aforesaid positive terminal. The capacitor C202 isconnected to the base of a transistor 0204 and through a 3.3--kilo-ohmresistor R215 to the lead B from the source voltage detector l6.

The emitter of the transistor 0204 is grounded, and its collector isconnected to the base of an N PN transistor 0203 and through a 15kilo-ohm resistor R203 to the positive terminal of the 15-volt source.The emitter of the transistor 0203 is grounded, and its collector,together with the diode CR206 is connected to the output lead Dextending to the coupling network 18.

Prior to the generation of a pulse by the pulse generator circuit 12 ofF IG. 1, the pre-amplifier l0 supplies current into the base of thetransistor 0209 maintaining the transistor 0209 in a conductivecondition. Thisholds the base of the transistor 0205 at groundpotential, so that the latter transistor is non-conductive. The base ofthe transistor 0201 is now held positive by the resistor R205, and thetransistor 0201 is also nonconductive. Current flows through theresistor R204 and through the Zener diode CR207, and through theresistors R210 and R211. The capacitor C202 is then charged toapproximately the Zener voltage of the Zener diode CR207 which may, forexample, be of the order of 5.1 volts.

The resistor R215 supplies base current to the transistor 0204 causingthe transistor 0204 to be conductive. The resistor R203 supplies basecurrent to the transistor 0203 only when the transistor 0204 isnonconductive. When transistor 0203 is non-conductive, the diode CR206does not conduct current. When the preamplifier 10 ceases to supplycurrent to the base of the transistor 0209, the resistor R214establishes the base at ground potential, causing the transistor to benon-conductive. When the transistor 0209 is non-conductive, currentflowing through the resistor R210 flows into the base of the transistor0205, causing the latter transistor to become conductive. When thetransistor 0205 becomes conductive, it supplies base current to thetransistor 0201 which, in turn, adds current to the base of thetransistor 0205, resulting in a regenerative action by which thetransistors 0205 and 0201 become fully conductive.

Current through the emitter of the transistor 0201 and through the diodeCR203 causes the lead R (connected to the resistor R204, to thecapacitor C202, to the diodes CR203 and CR206 and to the Zener diode207) to go negative. This, through the capacitor C202, causes thetransistor 0204 to become non-conductive, and causes the transistor 0203to become conductive, thereby initiating an output pulse. As thetransistor 0203 becomes conductive, the diode CR206 conducts and forcesthe lead R to approximate ground potential. The transistor 0203 remainsconductive until the capacitor C202 is discharged through the resistorR215 sufficiently to permit the base of the transistor 0204 to swingpositive so as to render the transistor 0204 conductive, thereby forcingthe transistor 0203 to its nonconductive state and terminating theoutput pulse.

Since the voltage charge level of the capacitor C202 is constant, theduration of the pulse generated by the circuit of FIG. 2 is a functionof the current in the resistor R215, which is approximately proportionalto the voltage on the lead B from the source voltage detector 16. Whenthe output pulse of the circuit of FIG. 2 is terminated, the loadvoltage is increased by the energy released to the filter capacitor C ofFIG. 1, so that the pre-amplifier again supplies current to the base ofthe transistor 0209, and this prevents a new pulse from occurring untilit is required.

Before a new pulse is generated by the circuit of FIG. 2, the capacitorC202 is charged through the resistor R215 to the voltage level of theZener diode CR207. If the inhibit control circuit 14 forces the lead Cto ground potential, a new pulse cannot occur, since the same conditionis created in the circuit as was created by the conductivity of thetransistor 0209.

The inhibit control circuit, as shown in FIG. 3, includes a diode CR212which is connected to the power diode CR of FIG. 1. The diode CR212 isconnected to the base of an NPN transistor 0207 and to a resistor R207.The resistor R207 may have a resistance of 15 kilo-ohms, and it isconnected to the positive terminal of the 15-volt source. The emitter ofthe transistor 0207 is grounded, and the collector is connected to aresistor R206. The resistor R206 may have a resistance of 47 kilo-ohms,and it also is connected to the positive terminal of the l S-voltsource.

The collector of the transistor 0207 is also connected to a capacitorC203 which, in turn, is connected to the base of an NPN transistor 0206and to a grounded resistor R212 which may have a resistance of IOkilo-ohms. The resistor R212 is shunted by a diode CR211. The emitter ofthe transistor 0206 is grounded, and the collector is connected to thepulse generator 12 over the lead C.

When the power diode CR is not conducting its cathode is positive withrespect to its grounded anode. In such a condition, the diode CR212 isnon-conductive and, the resistor R207 supplies base current to thetransistor 0207, holding the transistor conductive. Capacitor CR203 isdischarged. The resistor R212 holds the base of the transistor 0206 atground potential, and the transistor 0206 is non-conductive, so that thecircuit of FIG. 3 does not inhibit the pulse generator 12.

When the power diode CR conducts, however, its cathode becomes negativewith respect to the grounded anode, and when that occurs, the diodeCR212 is forward biased, which takes all the current of the resistorR207, so that the transistor 0207 is made non-conductive. Current nowflows through the resistor R206 and through the capacitor C203 to thebase of the transistor 0206, causing the transistor 0206 to becomeconductive, so that the lead C is established near ground potential soas to inhibit the operation of the pulse generator 12.

When the power diode CR ceases to conduct, the diode CR212 is againreverse biased, and the transistor 0207 is rendered conductive forcingthe base of the transistor 0206 negative through the capacitor C203,

and thus causing the transistor 0206 to become nonconductive.

The potential on the lead C now rises to a value so as to permit thepulse generator 12 to generate another pulse. If the power diodeconducts for a substantial time interval, the capacitor C203 continuesto charge as long as the transistor 0207 is non-conductive. When thecapacitor C203 is nearly completely charged, the current through theresistor R212 becomes equal to the current through the resistor R206,and there is no current available to the base of the transistor 0206, sothat the transistor 0206 becomes non-conductive permitting the pulsegenerator 12 to generate a pulse. This means that in the event anegative load causes a current to flow through the power diode CR duringa stand-by condition, the system will still operate when turned on.

Whenever the pulse generator 12 generates a pulse, the transistor 0 ofFIG. 1 controlling the power transformer T, is rendered conductive, andthe current through the power diode CR is removed. Then, the transistor0207 is rendered conductive, and the capacitor C203 is dischargedthrough the diode CR2ll, and normal operation resumes.

An appropriate circuit for the voltage source 20 and source'voltagedetector 16 is shown in FIG. 4. The alternating-current input isconnected to a group of diodes designated CR107, CR108, CR109 and CR110.Appropriate capacitors C19 and C20 are connected across the rectifier,and the resulting rectified power is applied to the primary N of thetransformer T The alternating-current input is also applied to theprimary of the transformer T The secondary winding of the transformer isconnected to a pair of diodes CR308 and CR310, the cathodes of which areconnected together and to the lead B so as to apply its output to thepulse generator 12. The center tap of the secondary winding of thetransformer T is grounded. Capacitor C106 is connected to lead B and toground.

The source voltage for the system may be obtained by directrectification of the altemating-current as shown in the upper circuit ofFIG. 4, the diodes CR107, CR108, CR109 and CR110 being connected in awell known bridge connection, and the capacitors C19 and C20 serving asfilter capacitors.

As shown in the lower part of the circuit of FIG. 4, the source voltagedetector includes a small transformer T whose primary is connectedacross the alternating-current input, and whose secondary is connectedto a full-wave 'center tap rectifier including the diodes CR307 andCR309, and the filter capacitor C106 which may, for example, be of theorder of 2,000 microfarads.

The values of the capacitors C19 and C20 and of the capacitor C106 arechosen so that the rate of discharge of the capacitors C19, C20 when theinput alternatingcurrent is removed is approximately equal to the rateof discharge of the capacitor C106, so that the voltage on the lead Bwill always track the voltage across the capacitors C19, C20proportionately. If the voltage on the lead B is properly chosen, forexample, at 15 volts, this voltage may also be used to supply theauxiliary voltage required by the circuits described above.

A complete circuit diagram for one embodiment of the power supply isshown, for example, in FIG. 5, and the circuit of FIG. includes theindividual circuits discussed above. In the circuit of FIG. 5, forexample, the capacitor C of FIG. 1 across the secondary N of thetransformer T is replaced by a group of 6,800 microfarad filtercapacitors C2-C5 and a series choke coil L is also included in theillustrated filter. Likewise, the power diode CR of FIG. 1 isrepresented in FIG. 5 by the power diode CR2.

As shown in the circuit of FIG. 5, the pre-amplifier comprises a PNPtransistor 0210 whose base is connected to a potentiometer R219 having aresistance of 250 ohms. The potentiometer R219 is connected to a 290 ohmresistor R218 and to a grounded resistor R222, the latter resistor alsohaving a resistance of 390 ohms. The emitter of the transistor Q210 isconnected to the junction of a Zener diode CR214 and a 470 ohm groundedresistor R220. The collector of the transistor 0210 is connected througha 2.2 kilo-ohm resistor R221 to the base of the input transistor Q209 ofthe pulse generator 12. A capacitor C201 of 6.8 microfarads is connectedbetween the l5-volt positive potential lead and ground. The Zener diodeCR214 and the resistor R218 are connected to the output of the powersupply, so as to permit the output voltage to be compared to a referenceand the difference to be amplified in the pre-amplifier 10 and appliedto the pulse generator 12 so as to prevent operation of the pulsegenerator, as mentioned above, so long as the output voltage of thepower supply is above a predetermined threshold value.

The source voltage detector of FIG. 5 also is used to supply the l5-voltauxiliary voltage to the auxiliary voltage lead R in FIG. 5. ResistorR216 is connected to resistor R215 and is used to adjust the normaloperating pulse width of the pulse generator.

In the circuit of FIG. 5, the switching transistor Q of FIG. 1 isreplaced by three NPN transistors Q2, Q3 and Q4. The emitters of thesetransistors are connected to the center tap of the secondary winding ofa transformer T The primary winding of the transformer T is connected tothe collector of a NPN transistor 0202 and to the cathode of a Zenerdiode CR209, the anode of which is grounded. The collector of thetransistor 0202 is also connected to a ground diode CR208, and theemitter of the tran-istor 0202 is connected to a grounded diode CR210.The cathode of the Zener diode is also connected to the cathode of adiode CR204 which, in turn, is connected through a 2.2 kiloohm resistorR201 to the l5-volt auxiliary voltage lead R. The output transistor Q203of the pulse generator 12 has its collector connected through a diodeCR205 to the base of the transistor 0202. The transistor 0202 and itsassociated circuitry constitutes the coupling network 18 of FIG. 1, andit causes the transistors Q2, Q3 and O4 to become conductive wheneverthere is a pulse output from the pulse generator 12, so as to charge thetransformer T The secondary of the transformer T is connected to thefilter capacitors C19 and C20 of the voltage storage source. The otherside of the secondary winding of the transformer T is connected to aseries of 47 microfarad capacitors C101, C102 and C103. The capacitorsC101, C102 and C103 are connected respectively to the base electrodes ofthe transistors Q2, Q3 and Q4. The secondary winding of the transformerT is also connected through a diode CR103 to three resistors R101, R102and R103, the resistors being connected to the base electrodes of therespective transistors Q2, Q3 and 04. I

The coupling circuit described above responds to each pulse output fromthe pulse generator 12 to render the transistors Q2, Q3 and Q4conductive, so that current may flow .from the voltage source 20 intothe transformer T, for the reasons described above.

The invention provides, therefore, an improved re gulated power supplysystem for converting alternating current into direct current. The powersupply system of the invention is light and efficient, and is relativelyinexpensive in its construction. The power supply system is capable ofmaintaining voltage regulation up to full load, and has a fallingregulation characteristic for overload conditions, so that the solidstate elements of the system are protected. The energy stored in thevoltage source capacitors is sufficient to permit the power supply tocontinue to transfer energy to the output capacitors and thus maintainregulation after an alternating current power failure for a timesufficient to permit the transfer of any information being processed inthe equipment powered by the power supply.

It will be appreciated, of course, that although a particular embodimentof the invention has been shown and described, modifications may bemade, and it is intended in the claims to cover the modifications whichcome within the spirit and scope of the invention.

What is claimed is:

l. A power supply system comprising: an electromagnetic unit comprisinga transformer having a primary winding and a secondary winding; aunidirectional voltage source coupled to said primary winding of saidelectromagnetic unit; a switch interconnected between said voltagesource and said primary winding; an output circuit coupled to saidsecondary winding of said electromagnetic unit and including aseries-connected unilaterally conductive device; a pulse generatingmeans coupled to said switch for introducing a train of pulses to saidswitch periodically to close said switch so as to cause current to flowfrom said voltage source into said electromagnetic unit thereby tocreate a unidirectional voltage across said output circuit of a polaritysuch that said unilaterally conductive element is not conductive, andperiodically to open said switch to cause unidirectional voltage acrosssaid output circuit to reverse so that unidirectional current isreleased from said electromagnetic unit to said out put circuit of apolarity to be conducted by said unilaterally conductive element;control circuit means having an input coupled to said output circuit andhaving an output coupled to said pulse generating means to prevent saidpulse generating means from generating a pulse when the voltage acrosssaid output circuit is above a predetermined threshold; a source voltagedetector circuit having an input coupled to said voltage source forproducing a signal proportional to the voltage of said source, andnetwork means included in said pulse generating means responsive tovoltage from said source detector circuit to vary the duration of theindividual pulses generated by said pulse generating means in an inverseproportional relationship with the amplitude of said signal from saidsource detector, so as to cause the product of the duration of theindividual pulses generated by said pulse generating means and theamplitude of the voltage from said source to be maintained constant;inhibit control circuit means coupled to saidunilaterally conductiveelement and to said pulse generating means to prevent said pulsegenerating means from generating a pulse so long as said unilaterallyconductive element is conducting, said pulse generating means includingcapacitive means which is charged to a predetermined value before a newpulse is generated thereby, and said inhibit control circuit means beingconnected to said capacitive means to prevent said capacitive means fromacquiring a charge so long as said unilaterally conductive device isconducting.

'2. The power supply system defined in claim 1, in

which said capacitive means is connected to a resistor in the aforesaidnetwork means, the current in said resistor being proportional to thevoltage from said source detector circuit, and the duration of eachpulse generated by said pulse generating means being a function of thecurrent in said resistor.

3. The power supply system defined in claim 1, in which said voltagesource includes a diode bridge circuit for rectifying analternating-current input, and filter capacitor means connected acrosssaid diode bridge circuit; and in which said source voltage detectorcircuit includes a transformer having a primary winding for receivingsaid alternating-current input, and a full-wave center tap rectifiercircuit connected to the secondary winding of the transformer.

4. The power supply system defined in claim 3, in which said full-wavecenter tap rectifier includes a filter capacitor having substantiallythe same rate of discharge as said first-named filter capacitor means sothat the voltage output of the source voltage detector circuit willtrack the voltage output of said voltage source proportionately.

1. A power supply system comprising: an electromagnetic unit comprisinga transformer having a primary winding and a secondary winding; aunidirectional voltage source coupled to said primary winding of saidelectromagnetic unit; a switch interconnected between said voltagesource and said primary winding; an output circuit coupled to saidsecondary winding of said electromagnetic unit and including aseries-connected unilaterally conductive device; a pulse generatingmeans coupled to said switch for introducing a train of pulses to saidswitch periodically to close said switch so as to cause current to flowfrom said voltage source into said electromagneTic unit thereby tocreate a unidirectional voltage across said output circuit of a polaritysuch that said unilaterally conductive element is not conductive, andperiodically to open said switch to cause unidirectional voltage acrosssaid output circuit to reverse so that unidirectional current isreleased from said electromagnetic unit to said output circuit of apolarity to be conducted by said unilaterally conductive element;control circuit means having an input coupled to said output circuit andhaving an output coupled to said pulse generating means to prevent saidpulse generating means from generating a pulse when the voltage acrosssaid output circuit is above a predetermined threshold; a source voltagedetector circuit having an input coupled to said voltage source forproducing a signal proportional to the voltage of said source, andnetwork means included in said pulse generating means responsive tovoltage from said source detector circuit to vary the duration of theindividual pulses generated by said pulse generating means in an inverseproportional relationship with the amplitude of said signal from saidsource detector, so as to cause the product of the duration of theindividual pulses generated by said pulse generating means and theamplitude of the voltage from said source to be maintained constant;inhibit control circuit means coupled to said unilaterally conductiveelement and to said pulse generating means to prevent said pulsegenerating means from generating a pulse so long as said unilaterallyconductive element is conducting, said pulse generating means includingcapacitive means which is charged to a predetermined value before a newpulse is generated thereby, and said inhibit control circuit means beingconnected to said capacitive means to prevent said capacitive means fromacquiring a charge so long as said unilaterally conductive device isconducting.
 2. The power supply system defined in claim 1, in which saidcapacitive means is connected to a resistor in the aforesaid networkmeans, the current in said resistor being proportional to the voltagefrom said source detector circuit, and the duration of each pulsegenerated by said pulse generating means being a function of the currentin said resistor.
 3. The power supply system defined in claim 1, inwhich said voltage source includes a diode bridge circuit for rectifyingan alternating-current input, and filter capacitor means connectedacross said diode bridge circuit; and in which said source voltagedetector circuit includes a transformer having a primary winding forreceiving said alternating-current input, and a full-wave center taprectifier circuit connected to the secondary winding of the transformer.4. The power supply system defined in claim 3, in which said full-wavecenter tap rectifier includes a filter capacitor having substantiallythe same rate of discharge as said first-named filter capacitor means sothat the voltage output of the source voltage detector circuit willtrack the voltage output of said voltage source proportionately.